What it actually is
Some polymers form a loose network at rest. Flow pulls that network apart and aligns the chains in the direction of movement, so resistance drops. Stop the flow and the network rebuilds, often within a second. The material has not been damaged; it has been temporarily rearranged.
Why it matters
It is why a dressing pours easily from the bottle and then sits on the salad instead of running to the bottom of the bowl, and why a sauce can be pumped through a line without losing the body it needs on the plate. Thickness at rest and thinness in motion are both required, and only this behaviour gives both.
What changes it
The material above all: xanthan is strongly shear thinning, guar much less so, and a native starch not at all in the same way. Concentration and the shear rate applied also matter, which is why a laboratory viscometer reading can mislead about line behaviour.
